How many structures of hydroxycarboxylic acid (molecular formula ${\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_3$) can show optical actvity?

Statement-1:  are stereoisomers.

Statement-2: Stereoisomers have different orientation of groups in space.

How many lone pairs of electrons are there in the given compound?

Decreasing order of $+ \mathrm{M}$-effect of given groups is :

$\left(\mathrm{I}\right)-{\mathrm{NR}}_2$  $\left(\mathrm{II}\right) -\mathrm{OCOR}$ $\left(\mathrm{III}\right) - \mathrm{NHCOR}$ $\left(\mathrm{IV}\right)-\mathrm{Ph}$